The TriMet system consists of a fleet of ~683 Buses operating on ~77 lines, Regional Light Rail (MAX), and Westside Commuter Rail (WES). The City of Portland operates a streetcar system in the downtown area. Additionally, there is an Aerial Tram that operates between Oregon Health Science University (OHSU) and South Portland.
You can ride MAX with your valid TriMet fare (HOP Card).
You can ride the Portland Street Car with your valid TriMet fare. Or you can ride only the Streetcar using their fare system.
$8 for a two-way ride. An annual or monthly TriMet or Streetcar pass will get you on for free.(Need to verify this is still true.) All others (eg tourist, thrill seekers) must pay to help pay for the operation of the Tram. One way fare not available for purchase. HOP Card is not valid. Please check the Portland Tram page, link below.
CC Rider (Coast), C-TRAN (Vancouver, WA), Vine (?), SMART (Wilsonville), CO-breeze (Bend), AMtrak. Also, there is tourist bus to the Gorge, and you can take Sandy Transit to Mt Hood and Timberline Lodge.
See Get In and Get Around on this great page for descriptions and links: Wiki Travel